Chapter 1 /S^ >0^s~^^O> 



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INTRODUCTION \5 \ ^^S^^ ^ I ^ 



The living world 



Biochemistry, as the name implies, means the chemistry of living 

 things. Obviously such a meaning includes the chemistry of plants and 

 microorganisms, as well as animals. The first two groups are indispensa- 

 ble to a living world, but the third is not. Although a living world com- 

 posed only of plants and microorganisms would be unfamiliar to us, it 

 would be adequate to maintain a balance between the synthetic processes 

 of the plant and the degradative processes of microorganisms. Put in 

 other terms, the carbon and nitrogen cycles in nature could be kept in 

 balance without the help of animals. The, latter are superimposed upon 

 the plants and microorganisms; and man, because of his dominant posi- 

 tion in the living world, places himself at its center. 



The brief phrase, "chemistry of living things," covers a vast field of 

 subject matter. It includes, in the first place, the chemical make-up of all 

 the individual substances of which living tissues are composed. These 

 substances are extraordinarily numerous. A single cell of the simj^lest 

 type contains scores, probably hundreds, of different chemical substances 

 — no one knows how many in any particular organism. Furthermore, 

 many of these substances, or compounds as the chemist prefers to call 

 them, are extremely complex. Whole classes of biological compounds 

 are so involved that, even today, the exact structural formula of no 

 single member is known; prime examples are the proteins and nucleic 

 acids. Quantitatively, the most important single constituent is water. 

 Everything else is classified as dry matter or solids, which consist mostly 

 of organic compounds (substances containing carbon), although many 

 inorganic substances are present in small amounts. 



Secondly, the "chemistry of living things" includes whatever chemical 

 changes the above substances undergo as the organism grows, reproduces, 

 absorbs and uses food, excretes waste products, and in general carries 

 out the activities incidental to being and remaining alive. The sum 

 total of all these chemical processes and the chemical compounds involved 

 in them is the living organism. The individual at any moment is a 

 dynamic balance between opposing processes of building up and break- 

 ing down, of taking in and throwing off, just as a lake is the resultant 

 of the inflow and outflow of its waters. 



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